Live Zika was not detected in tears during the trial window; however, its viral genetic material was found in the tears and lacrimal gland

Zika virus, a flavivirus typically spread via infected mosquitoes, causes congenital abnormalities, such as infant microcephaly, and Guillain-Barre syndrome. Its persistence in the eye raises questions about a new transmission vector as agencies try to control Zika’s spread, now stateside. This latest study helps explain why some Zika patients develop eye conditions, such as conjunctivitis and uveitis.

To determine Zika’s effects on the eye, researchers simulated an infected mosquito bite on adult mice. Those mice were found to harbor the virus in their eyes up to seven days; however, baby mice born to those infected mothers did not have the virus in their eyes.

That said, intentionally infected neonatal mice did show evidence of Zika once they were born. Of note, researchers found Zika RNA in the tears of infected mice weeks after initial infection. It’s unclear if there’s a point at which the tears could be infectious.

“Even though we didn’t find live virus in mouse tears, that doesn’t mean that it couldn’t be infectious in humans,” says study lead author Jonathan Miner, M.D., Ph.D., in a news release. “There could be a window of time when tears are highly infectious and people are coming in contact with it and able to spread it.”

Researchers suggest the eye may act as a reservoir for Zika. That’s because the eye’s immune system is naturally less active to avoid damaging visual tissue, and as a result, infections can persist there even after purging elsewhere in the body, according to the school.

Complementary studies using infected humans are in the works, researchers say. The study’s results could validate epidemiologists’ fears that tears are another bodily fluid contributing to fast transmission rates.

Zika study: What’s the key takeaway?

In a conversation between AOA Consultant Andrew Morgenstern, O.D., and the study’s lead author, Dr. Miner found it important to relay the following information directly to AOA members and the nation’s primary eye care providers:

Zika's effects in the human eye are presently unknown. A recent New England Journal of Medicine paper reported a human patient with Zika in the eye, resulting in uveitis, and humans with Zika frequently develop conjunctivitis. Dr. Miner says, "Whether Zika commonly infects the human eye is not known, but congenitally infected neonatal humans have severe eye pathology and blindness. This suggests that Zika does have the capacity to infect the human eye."

Study shows Zika in mouse eyes can be transmitted. This study detected live Zika in the eyes of laboratory mice, and that the virus could be transmitted to other mice. It also demonstrated infection of multiple structures, including the cornea, retina and optic nerve.

No evidence that Zika can be transmitted via tears. This study showed live Zika was not detected in tears during the trial window; however, its viral genetic material was found in the tears and lacrimal gland, suggesting that the virus is shed in tears. There is currently no evidence that Zika is transmitted by tears, Dr. Miner says, and human studies are required to confirm whether the virus is found in human tears and whether this may be another route of transmission.

"Mouse models tell us what may be possible in humans, but they do not tell us what actually happens in human patients," Dr. Miner stresses.

What eye doctors should know about Zika

Although the vast majority of laboratory-confirmed Zika cases result from transmission abroad, health authorities have identified two areas of Miami-Dade County where Zika is actively being spread by mosquitoes. To date, the Centers for Disease Control and Prevention (CDC) confirm 2,964 cases of travel-related Zika infection in every state, save Alaska and Washington, and 43 locally acquired cases—all confined to Florida.

Doctors of optometry should be especially aware, considering this latest research. While most patients with Zika exhibit no symptoms, some will experience a mild illness characterized by fever, rash, joint pain and/or conjunctivitis. The CDC offers key considerations for health care providers.

This study also states, "Viral infections in the eye can cause inflammation of uveal tissues (retina, choroid, iris and ciliary body), also termed uveitis, which can lead to permanent vision loss if untreated ... (Zika) causes conjunctivitis in 10-15 percent of infected adults, and uveitis occurred in a patient several weeks after initial infection. Fluid sampled from the anterior chamber of this patient's eye contained viral RNA (Furtado et al., 2016), suggesting that (Zika) can replicated within the eye at some stage of its clinical syndrome."

Should patients present with these symptoms, or those of Guillain-Barre syndrome, and if they—or their partner—traveled to a country known to have Zika, doctors should counsel regarding the Zika threat. Zika is a nationally notifiable condition requiring health care providers to report suspected cases to local, state or territorial health departments, the CDC notes.

Doctors of optometry can find relevant information pertaining to Zika and other infectious viral diseases at the following links: